The oxygen-evolving complex (OEC) of Photosystem II (PS II) in green plants, algae, and cyanobacteria contains a cluster of four manganese atoms as the catalytic site of photo-induced oxidation of water to oxygen. Manganese K-edge X-ray absorption spectroscopy (XAS) studies have been used to help elucidate the structure of this active site as well as the electronic environment around the Mn ions as the OEC accumulates oxidative equivalents. Although chloride was identified as a required cofactor for photosynthetic oxygen evolution over fifty years ago, the location of the chloride cofactor binding site is still unknown. It is thus desirable to probe the halide site directly and clarify its possible proximity to the Mn cluster. This proposal seeks to apply x-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure (EXAFS) at the chlorine edge in PS II membranes to determine whether Cl- is bound directly to Mn, and its role in the mechanism of photosynthetic water oxidation. Bromide and iodide can be substituted with some (albeit subdued) retention of OEC activity, and thus supporting experiments using Br- and I-- substituted PS II membranes are included in the experimental program.